Interesting reverb problem

[macula56]

I just rebuilt a reissue Vibrolux Reverb from the ground up. It sounds great but i am having an issue with the reverb and was wondering if anyone had seen this before. First off all everything is definitely wired up properly with all new components so I don't think that is the problem. At about 3 on the reverb dial a high whistling starts. As I turn it up to about 5 motorboating kicks in. If I keep turning the knob the motorboating frequency gets lower. While this is going on the overall B+ drops about 20 volts and the power tube bias goes thru the roof. If i pull the AT7 it has no effect on the problem so I am thinking it's in the recovery circuit. And as long as I keep the reverb knob at 2 or under everything sounds great. Reverb circuits are not my strong suit so I figured i would post this here as a lot of you guys have tons of experience with these circuits. Has anyone ever seen this before? Thanks, JMac.

[shooter]

5 motorboating kicks in

I've had this *issue* 3 times now in new builds, new parts, all 3 times it's been a low freq parasitic, mostly in the grounding section, causing a positive feed-back into my pre-amp section.  My *fixes* have been, snubber caps, ground separation, boosting dropping R's in the power-rail.  It's PITA scoping it, finding it everywhere and nowhere!

[macula56]

That part I get but I am at a loss about the voltage drop and the bias freaking out.

[sluckey]

I just rebuilt a reissue Vibrolux Reverb from the ground up.

Exactly what does that mean? Got a schematic and layout? Can you post some pics?

[macula56]

It means I took a reissue vibrolux and replaced everything in it. It's a dual channel Blackface based amp without the vibrato. Pretty standard stuff.  Other than the weird reverb problem the amp sounds great. A lot like my modded Deluxe Reverb that uses 6L6s. I can't post pics as I don't have a camera.

[sluckey]

What schematic and layout did you use?

[PRR]

> I am at a loss about the voltage drop and the bias freaking out.

You say it has major oscillation, both sub- and super-sonic. Oscillations rise to MAX output. The power amp is grossly over-driven (even if not too loud on the ear). Depending on topology and load, this can lead to increased DC current in the power stage or huge grid-clipping on power tube grids.

The recovery stage input is the MOST sensitive point in the amp. More gain than the guitar jack. If the least signal sneak-back happens from later stages, it will oscillate.

Examine and clean-up the grounding.

Re-consider the B+ filtering. Reverb stage B+ must be well isolated from power and driver B+. You generally can't take reverb driver and recovery both from the same B+ node.

[macula56]

I based it on the Blackface Deluxe reverb AB763 since that is what my Deluxe is wired like and it sounds great. So it's a typical AB763 schematic and layout. Without the vibrato part. Voltages are all pretty close to the schematic voltages. I will definitely look at where I took B+ from for both of these stages. I didn't know that made a difference.

[macula56]

> The power amp is grossly over-driven (even if not too loud on the ear). Depending on topology and load, this can lead to increased DC current in the power stage or huge grid-clipping on power tube grids.

How do I figure out what is over driving it so much? I am thinking that an Oscope would come in handy here.

[sluckey]

Does parasitic oscillation affect B+ or bias voltage?

It can. That's probably what's happening.

With the amp squealing use a gator clip jumper lead to ground V4 pin 2. Does that kill the squeal?

[macula56]

Does parasitic oscillation affect B+ or bias voltage?

It can. That's probably what's happening.

With the amp squealing use a gator clip jumper lead to ground V4 pin 2. Does that kill the squeal?

I'll try that when I get home. Thanks.

[macula56]

Does parasitic oscillation affect B+ or bias voltage?

It can. That's probably what's happening.

With the amp squealing use a gator clip jumper lead to ground V4 pin 2. Does that kill the squeal?

I'll try that when I get home. Thanks.

It kills the squeal and the reverb.

[sluckey]

Did you forget the 220K resistor that connects to pin 2? I would have been physically mounted on the reverb pedal jack in an original DR.

[macula56]

No Sir and it is grounded. I also unplugged the pan and it goes away too. But the pan is a known good unit as are the cables that attaches it. I tried 2 other good pans and still no change. I also checked the nodes and both tubes are getting B+ from the proper places and everything is grounded properly.

[PRR]

Ground the pan.

[macula56]

That didn't help either. Something new I just noticed, as I turn the reverb up the guitar volume diminishes almost like it's being dialed out of the circuit. This is in conjunction with the freq modulation.

[sluckey]

Perhaps you have a wiring error or wrong value component somewhere?

[macula56]

It turned out to be the cable from the output of the pan to the amp. It worked just fine in my other Deluxe but for some unknown reason would not work with this amp. I had already tried 3 good reverb pans and nothing worked. I can't explain it but when I changed out the cables everything went dead quiet like it should. I appreciate all of the helpful suggestions from PRR and Sluckey. Just goes to show that even though something works in one amp it may decide to crap out in the next one. Cables and pan should have been the first thing I changed but I could have sworn those cables were good. Go figure. Many thanks Gentlemen.

[PRR]

Check that cable's ground, end to end.